When a direct current potential is applied across an emulsion, suspension or solution containing charged particles or molecules, the latter migrate toward the electrode bearing the opposite charge. This phenomenon is generally called electrophoresis and is utilized to apply coatings onto metallic surfaces for purposes of electrical insulation, paint priming, weather protection, and the like.
Most of the polymer electrophoretic deposition techniques followed in industry involve aqueous systems. However, aqueous depositions are in general markedly affected and vitiated by the evolution of gases at the electrodes.
This is particularly true where polymer particles in a colloid or polymer molecules in a solution have a relatively low conductance. To cause such particles to migrate to and deposit on an electrode may require relatively high voltage which, in turn, causes the water to be electrolyzed and causes the evolution of hydrogen and oxygen as gases. This gas evolution, deriving from water electrolysis can result in heavily pitted polymer coatings, which makes them particularly unsuitable for electrical insulation. Moreover, water emulsion systems are generally plagued by difficult-to-control surface tension, necessary pH conditions, and viscosity difficulties.
Polyimide resins have recently come into use as high temperature electrical insulating films. Polymidide films are generally produced by film casting of a non-aqueous solvent solution followed by a heat cure. Briefly, this involves dissolving a suitable soluble polyamic acid precursor polymer in a solvent, casting the solution uniformly upon a smooth surface and then slowly heating until a suitable polyimide insulating film is produced from the acid polymer derivative. A major problem with the solution casting method is that only relatively thin films (0.0001 inch for foil coatings) can be produced in a single coat. Heavier coating or film thickness requires a multicoat system with each coat being dried and cured before applying the subsequent coat. Difficulty is also encountered with uniform coating of corners and particularly of sharp edges. Polyimide coverage of irregular-shaped objects has proved impossible by solution casting and expensive equipment costs are encountered in this method.
As a partial answer, electrophoretic deposition techniques have been developed for polyamic acids in a water emulsion system, as described in U.S. Pat. No. 3,537,970. Such an aqueous polymer electrodeposition system, however, still suffers the aforedescribed disadvantages of film pitting. Although most of the polymer electrophoretic deposition techniques applied in industry involve aqueous processes, a few organic systems, such as those shown in U.S. Pat. Nos. 3,450,655 and 3,463,714, have also been used. These systems have involved particulate colloidal suspensions of vinyl resins, epoxy resins, and carboxyl-containing polymers and copolymers such as polyacrylic acid, vinyl acetate/maleic acid copolymers, ethylene/itaconic acid copolymers and ethylene/maleic acid copolymers among others. Preparation of salt solutions of some of the foregoing carboxylcontaining polymers is described in U.S. Pat. No. 3,463,714 but the solutions are then converted into suspensions, and it is the suspended colloidal particles which are then electrodeposited.
It has been found that a great number of variables exist in non-aqueous electrodeposited systems as regards the ratio between polymer and solvent and between solvents within the solvent system, but that each polymer and system used presents its own characteristic problems in its dilution or suspension and deposition.